On page 4 you made reference to a study in 1989 that stated all fireplaces would spill,
during diedown, roughly at 10 pascals. I am curious what fireplaces were tested. So called
gasketed air tight doors? Machined fit doors?

I am also curious, is there not a thing as a truely air tight firebox? The
question keeps being raised how tight is tight?

Thank you for your time

John Viner

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From: Norbert SenfTo: John Viner

Hello John:

Please find a chart attached detailing the 5 fireplaces that
were tested.

So called gasketed air tight doors? Machined fit doors?

Commercially available doors at the time (1989)

I am also curious, is there not a thing as a truely air tight firebox? The question
keeps being raised how tight is tight?

The airtightness of the firebox is determined by the tightness of the loading doors,
plus any other similar hardware in the system such as cleanouts, dampers, etc. Also, with
metal chimneys there are the joints between the chimney sections.

Spillage rate is a function of the pressure difference between the room and the firebox,
and the size of leakage area. Obviously, with smaller cracks, there will be less leakage.

An important distinction to make is spillage RATE and spillage SUSCEPTIBILITY. Tighter
doors will reduce spillage rate, but not spillage susceptibility. Outside air can also be
shown to not reduce spillage susceptibility, since it does not lower firebox pressure,
relative to the room - this is the function of the chimney (if outside air lowered firebox
pressure it would, in fact, be a chimney - not a good thing.)

With wood smoke, even a few molecules are detectable by the human nose, since humans
evolved this as a survival function. Therefore, during startup, when there is smoke, you
could have a very tight door, and still detect smoke with your nose if the house pressure
is negative relative to the firebox. The actual smoke concentration could be minimal. Note
that this would be more of a nuisance factor than any real health or safety issue. The
best method to prevent poor startup draft is to avoid having the chimney on an outside
wall. Also, in my opinion, powered exhaust appliances in the house should be held
responsible for their own makeup air to avoid house depressurization.

From a safety viewpoint, the issue is spillage during the tailout phase, when there is
high CO and no smoke that can be easily smelled. This is more of an issue in systems with
rapid dilution air cooling such as open fireplaces, chimneys on outside walls, and systems
with low thermal mass, such as metal prefabs. Here, tight doors would reduce the potential
for CO concentration in the room tremendously.

Best ............... Norbert Senf

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Reply from John Gulland, who is one of the recognized authorities in this area:

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Norbert, John,

I never miss an opportunity to help people (especially people who talk to the public or
develop regulations) get a better understanding of how natural draft systems function in
modern houses. The issue of outdoor air supplies and alternative approaches has
become something of a specialty of mine. Further comments below.

The fireplaces ranged from loose (Oliver-McLeod) to extremely tight (Opel). These
differences made no difference in spillage performance: all five were fine at -5Pa and all
spilled during diedown at -10Pa.

> >I am also curious, is there not a thing as a truely air tight
> >firebox? The question keeps being raised how tight is tight?

John, you seem to be asking about the way fireplaces behave in a depressurized
environment. Norbert has given a good answer. In short, for batch fed
woodburning equipment, tightness only makes a difference in spillage volume, not spillage
susceptibility -- there is no such thing as a hermetically sealed woodburning
device. Following is an explanation of how Canadian code authorities responded to
the new information about outdoor combustion air supplies and natural draft combustion in
tight construction.

The 1990 National Building Code (NBC) of Canada had mandatory requirements for outdoor
air supplies for fireplaces, but, when the findings of Canada Mortgage and Housing
Corporation (CMHC) research on outdoor air supplies were tabled during the revision cycle
leading to the 1995 edition, the requirements were removed.

Here is some background on the evolution of outdoor air supplies in the NBC:

Appendix A-9.22.1.4. read in part: "The intent of this Article is to allow the
fireplace to be operated without affecting, or being affected by, other appliances or
exhaust equipment. For this to occur, the fireplace must be provided with a supply
of combustion air dedicated to the fireplace only; an opening to the exterior should be
provided at or near the fireplace opening."

The Article went on to require outdoor air for factory-built fireplaces in accordance
with the manufacturer's instructions and gave a series of prescriptive requirements for
outdoor air supplies for site-built masonry fireplaces.

In the proposed revisions sent out for public comment in August 1993, it was proposed
to delete Appendix note A-9.22.1.4. with the following reason given: "Combustion air
supplies as currently prescribed are generally ineffective. The requirement to
provide combustion air is being deleted from CAN/CSA A-405, Design and Construction of
Masonry Chimneys and Fireplaces and from the Code."

The '95 NBC contains the following: "9.22.1.4. Combustion Air. Where a
supply of combustion air is provided directly to the fire chamber of a fireplace,
including a factory-built fireplace, the installation shall comply with the "Outdoor
Air Supply" requirements provided by CAN/CSA A-405, Design and Construction of
Masonry Chimneys and Fireplaces." This is the only reference to combustion air
for fireplaces.

The supply of outdoor air was made non-mandatory and this wording was included because
the CMHC research that showed outdoor air supplies to be ineffective, also showed that
direct-to-combustion chamber supplies could be hazardous because of the potential for
wind-induced reverse flow of combustion gases through the supply duct. The A-405
requirements proposed ways to provide outdoor air safely if you choose to supply it.

Like most building codes in North America, the NBC had included outdoor combustion air
requirements for combustion equipment on the assumption that it was a good strategy to
reduce spillage susceptibility. Unfortunately the assumption was acted upon before
any research had been done to explore how outdoor air supplies actually behave.

The research reports that influenced the Standing Committee of Part 9 of the NBC are:

They are available from the CMHC information centre at (613) 748-2367.

Although the two studies were conducted by two labs with different set-ups, different
protocols and different appliance types (1. factory-built, 2. masonry), they arrived at
the same conclusion: The susceptibility to combustion spillage due to room
depressurization is not affected in a predictable way by the presence or absence of air
supplied from outdoors, whether supplied to the combustion chamber or indirectly through a
supply duct terminating near the fireplace.

In both studies the reference room depressurization at which spillage was induced was
10 Pa. In 'Fireplace Air Requirements', none of the five tested fireplaces spilled
at 5 Pa depressurization despite the fact that all were very different in their
configurations and features, although all did have glass doors. The tests at the two
depressurization levels were done with and without outdoor combustion air supplies.

Once the research findings were in and analyzed, the underlying physical process became
clear: That is, air flows to a zone of lower pressure through any available opening,
regardless of our wishful thinking. In retrospect, this principle appears rather
obvious, although for most of us it was not, until revealed in the lab.

As a result of the findings of these two studies, and against the backdrop of dozens of
other CMHC studies of combustion venting and building aerodynamics, it was recognized that
managing the indoor pressure environment was the only viable option for preventing health-
and life-threatening combustion spillage from chimney-vented atmospheric appliances.
This is particularly the case with automatic oil and gas equipment of this type because
they have dilution devices downstream of the combustion chamber: barometric draft controls
in the case of oil appliances and draft hoods in the case of gas appliances.
Dilution air cools the exhaust, weakens draft and offers a ready path for combustion
spillage, roughly equal to the spillage susceptibility of open fireplaces. Automatic
operation of gas and oil systems takes place independently of householder knowledge and
participation and may continue for long periods undetected and that is why this type of
spillage is considered potentially life-threatening.

But it was also recognized that hand-fed controlled combustion woodburning equipment do
not use dilution devices and have high spillage resistance during most operational periods
except for the tail out of the coalbed phase of the fire as system temperatures
cool. This type of spillage cannot continue for long without householder
intervention (reloading), and this implies awareness of any malfunction.

Therefore, the '95 NBC at 9.32.3.8. Protection Against Depressurization requires make
up air for exhausts exceeding 150 cfm where chimney vented oil and gas systems are
installed in the building. Where the only spillage-susceptible equipment present is
woodburning, the section requires only the installation of a carbon monoxide detector to
provide warning of spillage should it occur. A performance alternative to the
prescriptive approach of 9.32.3.8. is offered in the form of a reference to the CSA
ventilation code F326.

Hope this is useful. Let me know if you have any questions.

Regards,
John Gulland
The Wood Heat Organization Inc.www.woodheat.org
A non-commercial service in support of responsible home heating with wood.